1. Field of the Invention
The present invention relates to a telephone system capable of being connected to an ISDN or both the ISDN and the public telephone network.
2. Description of the Related Art
In accordance with the development of the digital processing technology and communication technology, a demand of establishment of a direct connection by means of a digital communication between information processing units which are positioned away from each other has been arisen. As a communication network capable of performing such a digital communication, an ISDN has been developed.
However, since all of communication information is arranged to be transmitted in the form of a digital communication information, the widely-used analog signal I/O terminals (to be called "an SLT" hereinafter) such as usual telephones for the public telephone network cannot be connected to the digital ISDN to receive data.